Generally, temperature sensors are used for measuring a temperature of an exhaust gas. Such a temperature sensor may be used for detecting a temperature such as of an exhaust gas that flows through a flow path. For example, such a temperature sensor may be used being arranged in a catalytic converter of an exhaust gas purification system or an exhaust pipe of a vehicle.
An example of such a temperature sensor is shown in FIG. 9. In FIG. 9, a temperature sensor 9 includes a temperature sensing element 90 composed of a thermistor 91 having electrical characteristics that change with temperature and a pair of metal electrodes 92 formed on the surfaces of the thermistor 91, and includes a sheath pin 95 incorporating signal lines 93 which are connected, on a tip-end side, to the temperature sensing element 90 and electrically connected, on a rear-end side, to an external circuit.
In a temperature sensor having such a configuration, the signal lines 93 are bonded, such as by welding, to the respective metal electrodes 92 which are bonded to the respective surfaces of the thermistor 91. Thus, the electrical characteristics of the thermistor 91 are detected by the external circuit.
A temperature sensor used in a high-temperature oxidizing atmosphere, such as in a catalytic converter or an exhaust pipe, has problems to be solved, i.e. of ensuring bonding reliability of the temperature sensing element that is a bonded member composed of the metal electrodes and the thermistor and of ensuring heat resistance of the metal electrodes.
As a measure against these problems, a bonded member composed of ceramic and metal is suggested (see PTL 1). This bonded member includes a metal film bonded to a surface of a ceramic material and a surface layer (oxide layer) formed on a surface of the metal film. PTL 1 also suggests a bonding method that includes diffusion bonding to achieve bonding between the metal film and the ceramic. PTL 1 teaches that the bonded member obtained in this way ensures its heat resistance and bonding reliability.
On the other hand, another bonded member is suggested, which is composed of ceramic and metal electrodes bonded to the surfaces of the ceramic (see PTL 2). The metal electrodes are provided as a continuous body and have a plurality of recesses. Similar to PTL 1, PTL 2 suggests a bonding method that includes diffusion bonding. PTL 2 teaches that the bonded member obtained in this way is able to reduce thermal stress which is ascribed to the difference in linear expansion coefficient between the ceramic and the metal electrodes and thus is able to ensure bonding reliability.
Further, a ceramic sensor is suggested, which includes a ceramics plate and a metal electrode bonded to at least one surface of the ceramics plate (see PTL 3). The metal electrode has an outer periphery which is entirely or partially cut off to entirely or partially expose an end of the ceramics plate. Also, at least a part of the outer periphery of the metal electrode has a thickness smaller than a center portion thereof. PTL 3 teaches that the ceramic sensor configured in this way is able to prevent the metal electrode from being separated.
Still another bonded member is suggested (see PTL 4 or 5). This bonded member is obtained by diffusing components of ceramics and metal into a bonding interface to ensure bonding reliability between the ceramics and the metal.
For example, PTL 4 suggests that, in bonding metal that contains Cr and Fe to nitride-base ceramics, the components contained in the ceramics are partially diffused into the metal to enhance bonding reliability. PTL 5 suggests that, in bonding ceramics, such as silicon nitride or silicon carbide, to metal that contains Cr and Ni, a silicide of Cr is formed in the interface between the ceramics and the metal to enhance bonding reliability.